Electron beam heating apparatus and control system therein

ABSTRACT

Electron beam heating apparatus is described in which the electron beam is deflected to a series of impact positions on a target surface. Signals for operating deflecting fields are produced by signal-generating means having provision for adjusting the duration at which the signal is at any given level. The energy applied to the target surface by the beam at any of the plurality of impact positions is thereby controllable by controlling the length of the time during which the beam is directed at each position.

United States Patent Inventors Appl. No. Filed Patented Assignee ELECTRON BEAM HEATING APPARATUS AND [56] References Cited UNITED STATES PATENTS 3,390,222 6/1968 Anderson 13/3 I 3,446,934 5/1969 Hanks 13/3 1x Primary Examiner-Bernard A. Gilheany Assistant Examiner-Roy N. Envall, .l A!t0rney-Anderson, Luedeka, Fitch, Even & Tabin ABSTRACT: Electron beam heating apparatus is described in which the electron beam is deflected to a series of impact positions on a target surface. Signals for operating deflecting fields ggiT ig T flT are produced by signal-generating means having provision for alms "wing adjusting the duration at which the signal is at any given level. US. Cl 13/31 The energy applied to the target surface by the beam at any of Int. Cl H05b 7/00 the plurality of impact positions is thereby controllable by Field of Search 13/31, 12; controlling the length of the time during which the beam is 2 1 9/1 21 EB directed at each position.

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sum 3 OF 3 oluvmc Purim STAGES FIG] ll mvnnoaa some" a. ANDERSON nun-r .0. xeuueov sauna nou ELECTRON BEAM HEATING APPARATUS AND CONTROL SYSTEM THEREIN This invention relates to electron beam heating and, more particularly, to-electron beam heating apparatus for heating a target surface and to a control system for use therein.

Electron beam furnaces of a variety of designs are useful in the processing of many metals, alloys or other materials, for

example where high standards of purity are to be achieved by outgassing or by avoiding reaction with oxygen and nitrogen, or where a substrate is to be coated by vaporization and condensation of the material. The electronbeams are a particularly useful formxof heating in that it is possible to inject heat into a melt locally. Electron beam furnaces typically include an evacuated enclosure, a container for the molten material being processed, andan electron beam heating system comprising one or more electron beam guns with associated deflection and control systems for directing and focusing the beams. i

Depending upon the particular type of processing being carried out, the container for the molten material may take a variety of forms. In a situation where it is desired to evaporate the material in the container and subsequently condense the material on a suitably supported substrate to coat the substrate, a typical container consists of an open-top upright crucible. Electron beam heating enables the crucible itself to be cooled and thereby form a skull of solidified molten material between the crucible and the molten material. This protects the purity of the molten material and makes it unnecessary to use high temperature refractories for the crucible construction.

Another type of processing is the purification of metals and alloys by passing the molten metal or alloy over a shallow hearth. Exposure to the vacuum with coincident electron beam heating of the surface causes many volatile impurities and occluded gases to be drawn off of the molten material and thereby produces a purified product. Other forms of containers which maybe utilized include tundishes, launders, and ladles for transferring molten material between various points. Electron beam heating may be utilized to maintain the material in a moltencondition while in such containers.

During the processing of molten material in an electron beam furnace, vaporized material may present ionization problems or may coat the various parts of the electron beam gun, impairing its operation. Moreover, spalling of condensed materials from cool surfaces of the vacuum enclosure, and

splashing andsplattering of the molten material from the crucible, may also impair operation of the'electron beam gun.

-By positioning the electron beam gun underneath the container of the molten material and by deflecting the electron beam through a curving path of 180 or more, contamination and shorting of the electron beam gun is minimized.

In electron beam furnaces in which large target surfaces are to be heated, it is often desirable to sweep the beam through a predetermined pattern over the target surface in order to minimize the number of electron beam guns required. In order to achieve the desired electron beam sweeping while deflecting the beam through more than 180, transverse magnetic fields which are variable in strength are typically utilized. If an orthogonal sweep pattern along xand y-axes is desired, two transverse fields in the electron beam path may be utilized. Such fields have, their flux lines oriented to cross the beam path generally perpendicular thereto and from directions which, in relation to the beam path, are effectively normalto each other.

Typically, magnetic field strength is varied by varying the current flowing through'the windings of the electromagnet used to establish the field. The position of the beam on the target along the particular axis which the field controls is then some function of the current that is applied to the electromagnet. When the beam is to be swept in a repetitive pattern, a

correspondingly repetitive current wave is applied to the electrornagnet. If a triangle wave is applied, a sweep will be inject more power into certain parts of the target than into other parts. This may be accomplished by generating a specially shaped wave to vary the sweep rate or direction of the beam, but it is often difficult to synthesize a wave of a complex shape that may be necessary for all possible heat patterns desired.

To alleviate the foregoing problem and thereby provide for varying the power injected into certain parts of a target without necessitating the generation of complex waves, control systems have been developed which synthesize the sweep wave digitally. This is done by producing a staircase waveform which approximates the analogue wave that is desired. Heretoforc known systems of utilizing this principle have varied the heat pattern by varying the concentration of digitally determined beam impact positions on a given section of the target surface as compared with other sections of the target surface. Since only a fixed. number of impact positions are typically available, however it is necessary to shift the total distribution of positions when it is desired to redistribute the heat pattern. Some limitations in achieving certain desired hcat patterns are encountered in such systems. Moreover, the necessity for changing the overall position distribution, when it is desired to change the heating pattern, introduces a considerable degree of inconvenience and complexity.

Accordingly, it is an object of the present invention to provide improved electron beam heating apparatus for heating a target surface.

Another object of the invention is to provide an improved control system for use in electron beam heating apparatus.

A further object of the invention is to provide a control system for use in electron beam heating apparatus in which the energy applied to the target surface by the beam may be applied in a wide variety of patterns.

It is another object of the invention to provide a control system for use in electron beam heating apparatus wherein the energy applied at the target surface by the beam at any of a plurality of digitally determined impact positions during a beam sweep is controllable in accordance with the length of the time during which the beam is directed at such position.

Other objects of the invention will become apparent to those skilled in the art from the following description, taken in connection with the accompanying drawings wherein:

FIG. 1 is a partially schematic and partially block diagram of electron beam heating apparatus incorporating the invention;

FIG. 2 is a schematic plan view illustrating a sequence of beam impact positions on the surface of a target for purposes of an example of one particular manner in which the invention may be operated;

FIGS. 3 and 4 are plots of current versus time illustrating examples of output signals which may be produced by the control system of the invention;

FIG. 5 is a schematic diagram of an adjustable pulse circuit which may be utilized in the system of FIG. 1;

FIG. 6 is a schematic diagram of a signal generator which may be utilized in the system of FIG. 1; and

FIG. 7 is a schematic diagram of a portion of a driving amplifier which may be utilized in the system of FIG. 1.

Very generally, the control system of the invention comprises signal-generating means 11 for producing at least one electrical signal of varying amplitude to cause the heating apparatus 12 to sweep an electron beam over a target surface 13. The electrical signal has a plurality of discrete levels representative of a corresponding plurality of beam impact positions on the target surface. Control means I4 are providedin the signal-generating means for controlling the time interval for which the signal is at any of the discrete levels.

Referring now more particularly to FIG. 1, the electron beam heating apparatus 12 incorporating the invention is illustrated. The illustrated target is a pool of molten material 16 which is heated by injecting heat into its exposed surface 13 by means of an electron beam, the envelope of which is indicated by the dotted lines 17. The molten pool 16 is contained within a crucible [Shaving a plurality of coolant passages 19 therein through which a suitable fluid coolant is circulated. Cooling of the crucible forms a skull 21 of solidified molten material between the molten pool 16 and the inner walls of the crucible 18. This aids in maintaining the purity of the molten material and in avoiding interaction between the molten material and the crucible material.

The electron beam 17 is produced by a suitable electron beam gun 22. in the illustrated system, the electron beam gun is located below the level of the surface 13 beneath the crucible 18 in order to protect the gun from evolved vapors and splattered material. The illustrated gun produces an electron beam 17 which is of a cross section approximating an elongated oval, such beam typically being referred to as a ribbonshaped electron beam. Such beams are produced by the use of an elongated emitter or filament from which the electrons are accelerated in a path which is normal or nearly normal to the filament. An electron beam gun of suitable construction for this purpose is shown and described in US. Pat. No. 3,170,0l9 assigned to the present assignee. it is to be understood, however, that the invention is not limited to the employment of such electron beam guns.

The electron beam is directed onto the surface 13 of the molten pool 16 by means of a suitable deflection system. The particular illustrated system is intended to be only one example of a system capable of sweeping the beam as described below. For example, other deflection systems capable of sweeping the beam are shown and described in US. Pat. Nos. 3,446,934; 3,235,647; and 3,390,222, all assigned to the present assignee. Step waveforms may be produced in accordance with the present invention to operate such deflection systems as described in the above patents, or other types of deflection systems as well. The nature'of such waveforms is dependent upon-the type of deflection system used and upon the desired sweep pattern.

The illustrated deflection system includes a pair of magnetic pole pieces 23 and a further pair of magnetic pole pieces 24, only one of which is visible. The pole pieces 23 are energized by the signal-generating means 11 by providing an electrical current through suitable energizing coils 25. The passage of current through the coils causes a magnetic field to be established extending between the pole pieces 23 transversely to the path of the electron beam 17. A magnetic field is established between the pole pieces 24 in a similar manner by appropriate energizing coils, not shown, to which electrical signals from the signal-generating means 11 are applied. It may be noted that-the direction of the magnetic fields extending between the pole pieces 23 and the pole pieces 24 are oriented 90 with respect to each other and the path of the electron beam. Variation in the strength of the magnetic fields between the pole pieces 23 and between the pole pieces 24 i may thereby be effected to cause movement of the beam along xand y-axes. By applying appropriate signals to the energizing coils for the deflection plates 23 and 24, the electron beam 17 may be swept in a pattern or raster over the surface 13 to effect heating of the molten material by injecting heat thereinto at the surface. Themagnetic field between the plates 24 is varied about a predetermined DC level in order to maintain a field strength sufficient to bend the beam through a curving path down onto the surface 13. The field between the deflection plates 23 and 24 also has a focusing affect on the electron beam to alter the beam cross section to an approximately round shape at the target surface.

In order to produce electrical signals of varying amplitude to cause the heating apparatus to sweep the electron beam over the target-surface, the illustrated signal-generating means 11 include a pair of deflection signal generators 26 and 27. The lateral deflection signal generator 26 provides signals through one channel of a dual-channel driving amplifier 28 to the energizing coils 25 for the pole pieces 23. The longitudinal deflection signal generator 27 provides similar signals through the other channel of the amplifier 28 for the unillustrated energizing coilsof the pole pieces 24. A preferred deflection signal generator for this purpose is described below, but other types of circuits for producing a step function voltage or current output in response to input control signals may be used under some circumstances. Accordingly, control over the electron beam on the respective xand y-axes is effected by changing the strength of the deflecting fields between fixed levels.

In order to provide control signals for the signal generators 26 and 27 and thereby cause the signal generators to change their output levels and thus move the electron beam to a'new position on the surface of the target, the signal-generating means include a series of pulse circuits 29, each havin'g afirst pulse output connected to each of the deflection signal generators 26 and 27 and a second pulse output connected to the next succeeding pulse circuit. The series of pulse circuits 29 form a ring counter. The adjustable nature of each of the pulse circuits 29 enables adjustment of the time between receipt of an input pulse from the immediately previous pulse circuit and the production of an output pulse at each of the two outputs. The first of the pulse circuits 29 is coupled to a start terminal 31. The output of the last pulse circuit is fed back to the input of the first pulse circuit. The number ofpulse circuits utilized corresponds to the number of discrete target positions desired for the beam sweep. The ring counter operation may be initiated by applying a suitable momentary pulse to the terminal 31, and maybe stopped by removing B+, not shown.

By way of example, and referring to FlG. 2, a circular target surface 13 is shown for molten material contained within a circular crucible 18. A lO-position sweep pattern is shown on the surface 13 designated by the spots labeled a-j. Assuming that a zero current output from the signal generator 26 and a 2-ampere current output from the signal generator 27 positions the beam at the spot j, the necessary output signals from the deflection signal generators 26 and 27 to sweep the beam through the positions from a toj are shown in FIG. 3 and FIG. 4, respectively. Thus, at position a, the output signal from the signal generator 26 is at O while the output signal from the signal generator 27 is at l ampere. At position b, the lateral output is moved to 0.2 whereas the longitudinal output moves to 1.5. The output signalscontinue in this manner until the beam is moved through a succession of positions ai and back to the position j once again. A feedback signal from the last pulse circuit to the first pulse circuit may start the sequence all over again. As will be explained, each change in current levels by the deflection signal generators 26 and 27 in accordance with a programmed pattern is initiated by receipt ofa pulse from a respective one of the pulse circuits 29.

In order to vary the residence time of the beam at any particular one of the lettered positions, the pulse circuits 29 are made adjustable by the control means 14. Thus, the length of time the output signals from the deflection signal generators 26 and 27 remain at any given level may be adjusted by suitable adjustment of the pulse circuits 29. By way of example, but not intending to be limited thereby, one possible way of utilizing the invention will now be discussed. in the sweep pattern of FIGS. 3 and 4, it may be desirable that the beam remain longer at the positions b, 0,7: and i than at the remaining positions, since the beam spot size may be larger at these positions due to the nature of the deflection system, with a corresponding reduction in heat transfer rate. By holding the beam longer at these regions of slower heat transfer, a more uniform temperature distribution in the molten pool may be achieved. On the other hand, the residence time required for the beam at the position j may be quite small, since the heat losses from the pool at this position are minimal and since the beam is sharply focused at this position. As may be seen in FIGS. 3 and 4, the length of the time during which the signals are appropriate for directing the beam to the positions b, c, i and j is 50 percent longer than the duration of the time during which signal levels are appropriate for directing the beam to the locations a, d, e, f and g. Moreover, the signals are at appropriate levels for directing the beam to the position j for a duration of time which is only one-third that of the time at which the beam is at positions b, c, h and i. Those skilled in the art will perceive that many variations are possible depending upon the particular heating pattern desired. By heating in this manner, the distribution of the various beam impact positions may remain the same, but the heat pattern may be radically changed by suitable variation of the time at which the beam is directed at any ofthe impact positions.

Referring now to FIG. 5, a particular circuit suitable for use as one of the pulse circuits 29 is shown. The pulse circuit is a unistable multivibrator followed by a trigger stage. The multivibrator portion of the pulse circuit includes a pair of NPN transistors 33 and 34 having common emitters. The transistors are connected in typical multivibrator fashion, with the collector of the transistor 33 being connected to the base of the transistor 34 through a capacitor 36, and with the collector of the transistor 34 being connected through a capacitor 37 to the base of thetransistor 33. The transistor 34 is normally biased to conduction from a positive voltage source 38 through a variable resistor 39 and a fixed resistor 41 connected to the base of the transistor 34. The transistor 33 is normally biased to cut off by connection through a resistor 42 to a source 46 of negative voltage. The resistor 43 couples the base of the transistor 33 to the collector of the transistor 34, and the resistor 44 connects the collector of the transistor 34 to thepositive'source 38. A collector resistor 47 is provided for the collector of the transistor 33 connected between the collector of the transistor and the positive source 38. The tap on the variable resistor 39 is connected to the positive source 38, and is provided with a filter capacitor 49 to ground. The emitter of the transistor 34 is connected through a filter capacitor 51 to the negative source 46, and the input terminal 52' of the multivibrator is connected through a diode 53 to the base of the transistor 33.

As previously mentioned, the transistor 34 is normally conducting and, therefore, the output of its collector is at reference or ground potential. Moreover, the transistor 33 is normally biased to cut off by the negative source 46. An input pulse of the shape indicated adjacent the terminal 52 is provided toinitiate triggering of the multivibrator. This pulse may be provided from the start terminal 31 (FIG. 1) or by feedback from the last pulse circuit (FIG. 1). In either case a positive pulse applied through the diode 53 to the base of the transistor 33 turns on the transistor 33. This effectively grounds the base of the transistor 34 through the capacitor 36 and the transistor 33, cutting off the transistor 34. The output at the collector of the transistor 34 is a square wave pulse, the width of which is determined by the off time of the transistor 34. The off time of the transistor 34 is determined by the control means 14, which is an RC circuit consisting of the capacitor 36 and the resistors 39 and 41. When the transistor 34 is turned back on due to the return of its base potential to positive,the voltage on the collector drops back effectively to ground, allowing the negative source 46 to bias the transistor 33 to cut off once again.

The square wave output at the collector of the transistor 34 produced by operation of the multivibrator is applied to the deflection signal generators as indicated by the arrow, and the shape of such wave is indicated adjacent the arrow. The width of'this wave determines the duration at which the respective signalgenerators 26 and 27 (FIG. 1) remain at the desired output levels. The width of this wave is regulated or controlled by varying the position of the tap on the variable resistor 39.

ln'addition to application to the deflection signal generators, the output signal of the multivibrator portion of the illustrated circuit is applied to a trigger circuit including an NPN transistor 56. The signal is applied to the base of the transistor 56 through a differentiator including a capacitor 57 and resistor 58. A diode 59 passes the negative portion of the differentiated pulse, and a coupling capacitor 61 applies this negative pulse to the base of the transistor 56. The junction between the diode 59 and capacitor 61 is connected to ground through a resistor 62. The transistor 56 is normally biased to conduction through a resistor 63 connecting the base of the transistor to the positive source 38. The transistor 56 conducts from the positive source 38 through a collector resistor 64 to ground, thereby maintaining the output at the collector of the transistor at reference or ground. Upon receipt of a differentiated pulse at the base of the transistor 56, the transistor is briefly cut off, causing the output at the emitter to pulse as indicated in the waveform adjacent thereto. The emitter is coupled to the next succeeding pulse circuit as shown by the arrow, or in the case of the last pulse circuit, it is coupled back (FIG. 1) to the input side of the first stage.

Although a particular circuit has been shown and described in connection with the pulse circuits 29, it is to be understood that any other suitably appropriate circuit may be utilized. Such circuit should provide for variable output time in order that the dwell time of the beam at any of the given impact positions is readily controllable. Accordingly, the energy applied to the target surface by the beam at any of the plurality ofimpact positions through which it is swept is controllable.

Referring now to FIG. 6, a portion of a signal generator which may be utilized as either the lateral deflection signal generator 26 or the longitudinal deflection signal generator 27 is illustrated schematically. The signal generator includes a series of transistor stages 66, each corresponding to a different one of the 10 beam positions a-j. The base of each of the transistors 66 is connected to a respective one of the pulse circuits 29, to the collector of the transistor 34 (FIG. 5) therein. This is true for the corresponding stages in both signal generators, so that each pulse circuit controls one stage in each signal generator. Preferably, the connection of the bases of the transistors 66 to the respective pulse circuits 29 is made through a suitable gate circuit which will remove any spurious pulses produced by the ring counter and thus ensure that the transistor stages 66 operate only one at a time.

Each of the transistor stages 66 has its collector connected to a power supply terminal 67 for supplying a positive voltage which is equal to or exceeds the maximum desirable output amplitude of the transistor 66. The emitter of each transistor 66 is connected to ground through a resistor 68 having a variable tap 69 thereon. The variable tap 69 of each resistor 68 is connected through a respective diode 71 to connect the portion of each of resistors 68 below the tap across a load resistor 72. The collector of each of the transistor stages 66 is grounded through a capacitor 65.

The signal developed across the load resistor 72 is connected through a variable resistor 73 to the base of an output transistor 74. The base of the transistor 74 is connected to ground through a capacitor 76, and the capacitor 76 and the variable resistor 73 form an RC circuit having an adjustable time constant. The collector of the transistor 74 is connected to the power supply terminal 67 and to ground through a capacitor 77. The emitter of the transistor 74 is connected through a resistor 78 to ground, and the output of the signal generator is derived at a variable tap 79 on the resistor 78.

In operation, each transistor stage 66 of the signal generator is triggered into conduction upon receipt of a pulse from the corresponding one of the pulse circuits 29, and is cut off upon termination of such pulse. The amplitude of the output of each of the transistor stages 66 is adjusted as desired for the corresponding one of the positions a-j by adjusting the tap 69. As each transistor stage 66 is turned on, its amplitude is set to the level in the step wave form corresponding to one position 0-], and when it is turned off and the next succeeding stage is turned on, the next stage is set to the level desired for the next level in the step function waveform. The RC circuit consisting of the variable resistor 73 and the capacitor 76 enables adjustment of the comers of the square output pulses provided by the transistors 66 to round the corners off and thus slow down switching and minimize spikes in the coil load. The variable tap 79 may be adjusted to adjust the amplitude of the total wave form output of the signal generator. When the termination of the output pulse of an adjustable circuit 29 coincides with the beginning of the output pulse of the next succeeding pulse circuit, the termination of the output of one of the transistors 66 coincides with the beginning of the output of the next succeeding transistor stage. Accordingly a step form output is generated in accordance with the settings of the taps 69.

Referring now to FIG. 7, the initial portion is shown of one of the two channels of the driving amplifier 28 of FIG. 1. The output of the appropriate signal generator is applied to one of the two input terminals 81 of the driving amplifier 28. The input terminal 81 is for one of the two channels of the driving amplifier, and the channels are identical. Accordingly, only the initial portion of one channel will be described. An initial transistor stage 82 is provided having its base coupled to the terminal 81 and its collector coupled to a plus Vl source 83 of positive potential. The collector of the transistor 82 is also coupled to ground through a capacitor 84. The emitter of the transistor 82 is coupled to ground through a resistor 86 and is also coupled to the juncture of a pair of Zener diodes 87 and 88. The cathode side of the Zener diode 87 is coupled through a resistor 89 to a plus V2 source 91 of positive potential which is substantially higher than the plus V1 source 83. The anode of the Zener diode 88 is connected through a resistor 92 to a minus V2 source 93 of negative potential of a magnitude corresponding to that of the plus V2 source 91. A capacitor 94 is connected across the Zener diode 87 and a capacitor 96 is connected across the Zener diode 88. The junction between the Zener diode 87 and the resistor 89 is connected through a resistor 97 to one end of a resistor 98. The junction between the Zener diode 88 and the resistor 92 is connected through a resistor 99 to the other end of the resistor 98. A variable tap 101 is provided on the resistor 98, and this tap is coupled through a resistor 102 to further amplification stages, not illustrated, in the particular channel of the dual-channcl amplifier 28.

The output of the signal generator is applied to the corresponding channel in the amplifier 28. The initial transistor stage 82 thereof isolates the remaining portions of the driving amplifier from the signal-generating circuitry. The two Zener diodes 87 and 88 provide a fixed reference voltage across the resistor 98. Thus, for example, the reference voltage with the tap 101 at the upper end of the resistor 98 may be 6.2 volts and the reference voltage with the tap 101 at the lower end of the resistor 98 may be minus 6.2 volts. With the tap positioned directly in the middle of the resistor 98, the reference voltage may be zero. By appropriately adjusting the tap 101, the step waveform output of the signal generator may be superimposed on a desired DC reference level to move this waveform to the desired relationship with respect to zero. Thus, although the actual output of the longitudinal deflection signal generator is of the shape shown in FIG. 3 but with the position a voltage at zero, by adjusting the tap 101 on the appropriate channel of the driving amplifier, a DC reference level of 1 a. may be achieved. This moves the waveform to the desired position as shown in FIG. 3. Similarly, by superimposing the output of the lateral deflection signal generator on a negative DC reference, the waveform of FIG. 4 may be produced, having negative portions even though a positive output only is obtained from the signal generator transistor stages 66 (FIG. 6).

The unillustrated amplification stages in the channel of the driving amplifier illustrated in FIG. 7 may be of any suitable type and will not be discussed in detail herein. For example, a suitable series of amplification stages may be provided terminating in a dual transistor push-pull type of output. Appropriate circuitry to provide the necessary amplification will be apparent to those skilled in the art.

Although the invention has been described in connection with a system wherein a ring counter having adjustable width pulse-producing stages is connected to drive the deflection system through a signal generator and amplifier, it may be possible to design satisfactorily operating systems wherein the deflection coils are driven directly by the ring counter. In such a system, the pulse width which governs the so-called dwell time, that is, the time interval for which the beam is directed at any one of the impact positions, is adjustable.

It may therefore be seen that the invention provides improved electron beam heating apparatus employing an improved control system therein for controlling the energy applied to a target surface by the electron beam. Beam sweeping is effected digitally, but the heating pattern may be readily changed by merely varying the dwell time of the beam at any of the various impact positions through which it is swept, without changing the distribution pattern of such positions.

Various modifications of the invention in addition to those shown and described herein will become apparent tothose skilled in the art from the foregoing description and accompanying drawings. Such modifications are intended to fall within the scope of the appendant claims.

What we claim is:

l. A control system for use with electron beam heating apparatus for heating a target surface, comprising, signalgenerating means for producing at least one electrical signal of varying amplitude to cause the heating apparatus to sweep the electron beam over the target surface, said electrical signal having a plurality of discrete levels representative of a corresponding plurality of beam impact positions on the target surface, said signal-generating means including control means for controlling the time interval for which said signal is at any of said discrete levels, whereby the energy applied to the target surface by the beam at any of the plurality of beam impact positions during a beam sweep is controllable in accordance with the length of the time during which the beam is directed at such position.

2. A system according to claim 1 wherein said signal generating means comprise a pair of signal generators generating respective coordinate signals, wherein each signal has a plurality of discrete levels, and wherein said control means are capable of varying the time intervals for both signals.

3. A system according to claim 2 wherein said signalgenerating means comprise a plurality of pulse circuits operable sequentially to cause said signal generators to change the amplitude of said signal sequentially from one of said discrete levels to the next.

4. A system according to claim 1 wherein said signalgenerating means include a ring counter comprising a series of one-shot multivibrator stages, each having means for varying the duration of its output pulse.

5. A system according to claim 1 wherein said signal generating means comprise, a pair of signal generators for generating respective deflection signals, each signal generator having a separate stage for generating one of a series of discrete levels comprising the output wave form of each said signal generator, a plurality of pulse circuits operable sequentially to activate in succession the stages of each of said signal generators, and wherein said control means comprise an RC circuit in each of said pulse circuits.

6. A system according to claim 5 wherein said signalgenerating means further include a dual channel drive amplifier having its respective channels connected to said signal generators, respectively.

7 Electron beam heating apparatus for heating a target surface, comprising, means for producing an electron beam, deflection means for deflecting the electron beam over the target surface, signal-generating means for producing at least one electrical signal of varying amplitude to cause the deflection system to sweep the electron beam over the target surface in a predetermined pattern, said electrical signals having a plurality of discrete levels representative of a corresponding plurality of beam impact positions on the target surface, said signal-generating means including control means for controlling the time interval for which said signal is at any of said discrete levels, whereby the energy applied to the target surface by the beam at any of the plurality of beam impact positions during a beam sweep is controllable in accordance with the length of the time during which the beam is directed at such position.

8. Electron beam heating apparatus for heating a target surface, comprising, means for producing an electron beam, x-

and y-axes deflection means for deflecting the electron beam in mutually perpendicular axes, a pair of signal generators, each connected to a respective one of said deflecting means for producing an electrical signal of varying amplitude to cause said deflecting means to sweep the electron beam in its associated axis, said electrical signals produced by said signal generators each having a plurality of discrete levels represen- 

1. A control system for use with electron beam heating apparatus for heating a target surface, comprising, signal-generating means for producing at least one electrical signal of varying amplitude to cause the heating apparatus to sweep the electron beam over the target surface, said electrical signal having a plurality of discrete levels representative of a corresponding plurality of beam impact positions on the target surface, said signalgenerating means including control means for controlling the time interval for which said signal is at any of said discrete levels, whereby the energy applied to the target surface by the beam at any of the plurality of beam impact positions during a beam sweep is controllable in accordance with the length of the time during which the beam is directed at such position.
 2. A system according to claim 1 wherein said signal generating means comprise a pair of signal generators generating respective coordinate signals, wherein each signal has a plurality of discrete levels, and wherein said control means are capable of varying the time intervals for both signals.
 3. A system according to claim 2 wherein said signal-generating means comprise a plurality of pulse circuits operable sequentially to cause said signal generators to change the amplitude of said signal sequentially from one of said discrete levels to the next.
 4. A system according to claim 1 wherein said signal-generating means include a ring counter comprising a series of one-shot multivibrator stages, each having means for varying the duration of its output pulse.
 5. A system according to claim 1 wherein said signal-generating means comprise, a pair of signal generators for generating respective deflection signals, each signal generator having a separate stage for generating one of a series of discrete levels comprising the output wave form of each said signal generator, a plurality of pulse circuits operable sequentially to activate in succession the stages of each of said signal generators, and wherein said control means comprise an RC circuit in each of said pulse circuits.
 6. A system according to claim 5 wherein said signal-generating means further include a dual channel drive amplifier having its respective channels connected to said signal generators, respectively.
 7. Electron beam heating apparatus for heating a target surface, comprising, means for producing an electron beam, deflection means for deflecting the electron beam over the target surface, signal-generating means for producing at least one electrical signal of varying amplitude to cause the deflection system to sweep the electron beam over the target surface in a predetermined pattern, said electrical signals having a plurality of discrete levels representative of a corresponding plurality of beam impact positions on the target surface, said signal-generating means including control means for controlling the time interval for which said signal is at any of said discrete levels, whereby the energy applied to the target surface by the beam at any of the plurality of beam impact positions during a beam sweep is controllable in accordance with the length of the time during which the beam is directed at such position.
 8. Electron beam heating apparatus for heating a target surface, comprising, means for producing an electron beam, x- and y-axes deflection means for deflecting the electron beam in mutually perpendicular axes, a pair of signal generators, each connected to a respective one of said deflecting means for producing an electrical signal of varying amplitude to cause said deflecting means to sweep the electron beam in its associated axis, said electrical signals produced by said signal generators each having a plurality of discrete levels representative of a corresponding displacement in the x- or y-axis, and pulse means connected to said signal generators and operable sequentially to cause said signal generators to change the amplitude of the signal produced thereby sequentially from one of said discrete levels to the next for deflecting the beam in a predetermined sweep pattern. 